Add BufWriter::with_capacity(64KB) to generate_test_file,
generate_growable_file, and append_lines in data_gen.rs.
Previously each writeln! triggered an individual write syscall,
making 5GB/74M-line benchmark data generation extremely slow.
BufWriter batches writes into 64KB chunks, reducing syscalls
by ~1000x.
Explicit flush()? + drop before subsequent reads ensures data
visibility and propagates flush errors (BufWriter::drop swallows
them).
Closes#35
6dd87d2872